Richard J Kinch wrote:

George E. Cawthon writes:

Air in the tank when it is filled
isn't going to increase the pressure over what it would be
if filled without any air

False.

Ok, so I exaggerated a bit since the total pressure is equal
to the total of the partial pressures. If you took all the
air in the tank at 1 atmosphere and compressed it into the
space above the liquid nitrogen (compressed bout 5 times?)
the psi would be pretty low and much lower than the propane
gas. Since gas volume and pressure are inversely related
and one atmosphere =14.7 psi, the pressure would be 5 x 14.7
or 73.9 psi. So just add that to the pressure of the
propane which someone suggest was at least 200 psi. I don't
think that has any significance for safety, unless they make
awfully weak bottles.

And when you use the
tank, you use gas, not liquid, so the air comes out which is
the problem.

True, eventually.